The aerodynamic performance of an airplane, at both low airspeed and high airspeed improves with an increase in aspect ratio (span.sup.2 /wing area) of the lifting surfaces, e.g., the main wing of the airplane. Theoretically, for a given lifting surface, the aerodynamic drag of the lifting surface and the airplane is reduced as the span of the lifting surface is increased. Therefore, a lifting surface with the lowest aerodynamic drag should have a long span and a relatively short chord. High-performance sailplanes are examples of aircraft having similar high aspect ratio lifting surfaces.
Commercial airline type airplanes have lifting surfaces which are designed for high airspeed flight of about Mach 0.8 and at much higher lifting surface area loading than high performance sailplanes. To achieve the high speed capability, the lifting surfaces are generally swept and the airfoil sections are thin. Since the lifting surfaces are highly loaded, the span has to be limited to accommodate the wing bending loads. To achieve a significant drag reduction on a commercial airplane already in production requires an increase in aspect ratio, i.e., an increase in span. Any span increase causes increased bending moments on the existing wing and requires an increase in the structural strength of the existing wing box. This strengthening can only be accomplished by increasing the structural thickness of the wing box members, such as skins, stringers, and spars.
A first approach for increasing the aspect ratio of a trapezoidal swept main wing of an airplane is to add a wing tip extension member to the outboard ends of the main wing. In this approach, the leading and trailing edges of each wing tip extension member is in alignment with the leading and trailing edges of the tapered main wing. The convergence of leading and trailing edges reduces the airfoil chord of the wing tip extension member. As a consequence, there is inadequate space for installing high-lift leading edge devices such as slats or flaps, known in the trade as "Krueger" flaps. The absence of a leading edge device on a wing tip extension member could produce undesirable effects, such as premature stall, on the wing tip extension member. This stall condition could further induce stall progression into the main wing thereby causing buffet, roll-off, and pitch-up of the airplane during low speed flight.
A second approach also uses wing tip extension members. In this case, however, each wing tip extension member has a leading edge in alignment with the leading edge of the swept main wing and a trailing edge whose sweep angle is greater than the sweep angle of the trailing edge of the main wing. This wing tip extension member has a larger tip chord than the tip extension of the first approach. Consequently, high-lift devices may advantageously be installed within each wing tip extension member. However, this second approach adds a relatively large incremental wing area to the basic wing area for a relatively modest increase in the overall aspect ratio of the main wing.
The increased area of the wing tip extension member of the second approach compared to the increased area provided by the first approach has other disadvantages, including a relatively large increase in: (1) weight, (2) aerodynamic friction drag, and (3) aerodynamic airloads on the extension members during gusts. The increase in aerodynamic airloads causes high wing bending moments requiring significant structural redesign and strengthening of the basic wing box from the root section of the main wing to the outboard section of the wing tip extension member.
A third approach for theoretically increasing the aspect ratio of a sweptback tapered main wing is the use of end plates or wing tip fins to significantly reduce wing tip vortices and the drag induced thereby. The wing tip fins are generally positioned vertically or at slight cant angles to the main wing chord plane. Their resultant lifting force is essentially sideways which can affect the yaw characteristics of a main wing. However, there are also wing tip fins that are positioned at a substantial outward cant angle where both side forces and vertical lifting forces are developed. Regardless of the angular position of a wing tip fin, an increase in lift is induced on the main wing surface immediately inboard of the fin.
An advantage of wing tip fins is that any increase in wing bending moments is generally less than the wing bending moments produced by wing tip extensions in the same chord plane as the main wing. This is due to the fact that the added lift generated on the main wing by the wing tip fins occurs further inboard than the added lift produced by wing tip extension members.
A disadvantage to the use of wing tip fins is that more wetted area is required, relative to the above-described wing tip extension members, to achieve a similar percentage reduction in induced drag. Also, analysis of the wing elastic characteristics indicates that wing tip fins of large size can cause wing flutter. The sharp corner formed at the intersection of a vertical wing tip fin and the main wing can cause additional aerodynamic problems, such as the formation of shock wave induced separation of the boundary layer airflow over the upper surface of the main wing immediately inboard of the fin thereby causing an early drag rise in that location. Finally, the sharp corner causes high local stresses requiring heavy machined forgings at the juncture of the wing tip fin and the main wing. These forgings are generally attached to the main wing spars by large bolts which makes the corner joint heavy and expensive.